This invention relates to a fuel injector for use in supplying fuel to a combustion space of a compression ignition internal combustion engine. In particular, the invention relates to an injector of the outwardly opening type. Such an injector is suitable for use in, for example, a common rail type fuel system.
A typical, known fuel injector of the outwardly opening type is shown in FIG. 1, and includes a valve needle 10 which is slidable within a bore 12, the valve needle 10 including a valve needle guide region 12a. The valve needle 10 is engageable with a seating 14, defined by a fuel injector nozzle body 16, to control the supply of fuel from the bore 12 to the cylinder of an associated engine, fuel being delivered through first and second outlet openings occupying different axial positions on the valve needle 10. The valve needle 10 is biased against the seating 14 by means of a spring 18, and by means of fuel pressure, and is movable away from its seating 14 by means of a piezoelectric actuator arrangement, only part of which is shown, which includes a piezoelectric stack which is arranged to control fuel pressure within a control chamber, defined, in part by a bore 22 provided in an upper housing part. A surface associated with the valve needle 10 is exposed to fuel pressure within the control chamber, an increase in fuel pressure within the control chamber causing the valve needle 10 to move outwardly within the bore 12 away from the seating 14, against the force of the spring 18 and the fuel pressure, and thereby permitting fuel within the bore 12 to flow into the engine cylinder through the one or more of the outlet openings depending on the extent of movement of the valve needle 10 away from the seating 14.
A problem with fuel injectors of this type is that, as the valve needle 10 moves away from its seating 14, fuel within the bore 12 is able to leak past the valve needle guide region 12a into the engine cylinder. This leads to a poor fuel spray characteristic which can result in an inefficient combustion of fuel and high levels of emissions. In addition, as the bore 12 is prone to dilate due to the high pressure of fuel within the bore, the level of fuel leakage from the bore can increase during the service life of the injector.
It is an object of the present invention to provide a fuel injector of the outwardly opening type which alleviates the aforementioned problems.
According to the present invention there is provided an outwardly opening fuel injector comprising a valve needle movable within a bore provided in a nozzle body and engageable with a seating to control the supply of fuel from the bore, the valve needle being biased towards its seating and being moveable outwardly of the bore to move the valve needle away from its seating, the fuel injector further comprising a sleeve member which is moveable with the valve needle, the valve needle and the sleeve member together defining a chamber for fuel such that, in use, fuel pressure within the chamber serves to dilate the sleeve member to reduce fuel leakage from the injector.
As the sleeve member dilates due to fuel pressure within the chamber, the seal between the nozzle body and the sleeve member is improved and fuel leakage from the bore is reduced or prevented.
The chamber may be supplied with fuel by a clearance passage which may be defined, at least in part, by formations, for example flats, slots, grooves or flutes, provided on the surface of the valve needle.
Conveniently, the sleeve member may be provided with first and second outlet openings occupying different axial positions on the sleeve member. In use, movement of the valve needle away from the seating into a first fuel injecting position may cause the first outlet opening to be exposed to permit fuel delivery through the first outlet opening, and movement of the valve needle away from the seating into a second fuel injecting position may cause the second outlet opening to be exposed to permit fuel delivery from both outlet openings.
First and second sets of outlet openings may be provided, each set including one or more outlet opening. The first and second outlet openings may be of different size or may provide a different fuel spray cone angle to permit the fuel injection characteristic to be varied, in use.
The sleeve member may be engageable with a further seating defined by the nozzle body, movement of the valve needle into the second fuel injecting position causing the sleeve member to engage the further seating. Thus, when the valve needle is moved away from its seating into the second fuel injecting position, fuel is unable to flow past the further seating. In this way, the seal between the nozzle body and the sleeve member is further improved and fuel leakage from the injector is further reduced.
Preferably, a surface associated with the valve needle is engageable with an additional seating defined by the bore to limit outward movement of the valve needle within the bore.
The surface associated with the valve needle may preferably be defined by a step in the surface of the sleeve member. The additional seating may preferably be defined by a step in the surface of the bore.
According to a second aspect of the present invention, there is provided an outwardly opening fuel injector comprising a valve needle movable within a bore provided in a nozzle body and engageable with a seating to control the supply of fuel from the bore, the valve needle being biased towards its seating and being moveable outwardly of the bore to move the valve needle away from its seating, a surface associated with the valve needle being engageable with an additional seating defined by the bore to limit outward movement of the valve needle within the bore.
Conveniently, the fuel injector may include a sleeve member which is movable with the valve needle, the sleeve member defining the surface which is engageable with the further seating.